1. Field of the Invention
The present invention relates generally to light receiving semiconductor devices and, more particularly, to anti-reflection structures for improving optical sensitivity of the light receiving semiconductor devices.
2. Description of the Background Art
Light receiving semiconductor devices are widely used as photosensors, photocouplers and so on, and, in many cases, are integrated in a common semiconductor chip on which a related signal processing circuit is also formed.
FIG. 1 is a schematic cross-sectional view showing one example of a conventional light receiving semiconductor device. For example, an N-type epitaxial layer 2 is formed on a P-type semiconductor substrate 1 and P-type isolation regions 3 are formed, reaching P-type semiconductor substrate 1 from the surface of N-type epitaxial layer 2 so as to separate N-type epitaxial layer 2 into a plurality of islands. One N-type epitaxial region 2a surrounded by P-type isolation region 3 and P-type substrate 1 constitute a light receiving element PD1 such as a photodiode, and semiconductor elements for a related circuit are formed in other N-type epitaxial regions surrounded by P-type isolation regions 3. While the surface of epitaxial layer 2 is covered with an oxide film 4 such as SiO.sub.2 which serves as an insulation film, oxide film 4 is eliminated on the surface in N-type epitaxial region 2a to be a cathode of light receiving element PD1.
The area where oxide film 4 on cathode 2a of light receiving element PD1 is eliminated is covered with a nitride film 5 such as Si.sub.3 N.sub.4 serving as an anti-reflection film. Oxide film 4, however, extends over the boundary of P-type isolation region 3 and into cathode region 2a by a prescribed small distance. That is, the peripheral portion of cathode region 2a is not covered with an anti-reflection film 5 but with insulation film 4.
In this regard, although anti-reflection film 5 of Si.sub.3 N.sub.4 on epitaxial layer 2 has a small reflectance of about 5%, insulation film 4 of SiO.sub.2 has a considerably large reflectance of about 15%.
Accordingly, in the structure of the conventional light receiving element PD1, light reflection cannot be efficiently reduced in the peripheral portion of N-type epitaxial region 2a which is not directly covered with anti-reflection film 5, reducing optical sensitivity of light receiving element PD1. In particular, such a decrease in the optical sensitivity causes a problem in a split photodiode used in an optical pick-up and so on. Construction of a split photodiode having six sub-regions is shown, for example, in N. Noguchi et al, "Preamplifier IC with Photodetectors for Optical Pickup" in The Institute of Electronics and Communication Engineers of Japan, Vol. 86, No. 241, pp. 75-88 published on Nov. 25, 1986.
FIG. 2 illustrates one example of a light receiving semiconductor device used in such an optical pick-up. Although two photodiode regions of a split photodiode device appear in this sectional view, the split photodiode device may include any number of photodiode regions. Though the semiconductor device of FIG. 2 is similar to that in FIG. 1, it includes photodiodes PD2 and PD3 formed in two adjacent islands 2b and 2c, respectively. Each of photodiodes PD2 and PD3 has the same structure as that of photodiode PD1 in FIG. 1.
FIG. 3 is a graph showing the optical sensitivity characteristic of the light receiving semiconductor device of FIG. 2 wherein the ordinates represent optical sensitivity and the abscissas represents positions on the semiconductor chip. The curved line PD2 represents the optical sensitivity characteristic of photodiode PD2 on the left side in FIG. 2 and the curved line PD3 represents the characteristic of photodiode PD3 on the right side. As seen from this graph, since the anti-reflection in the peripheral portions of photodiodes PD2 and PD3 is insufficient, the optical sensitivity is lowered in the vicinity of the boundary of didoes PD2 and PD3 as indicated by the dotted line (PD2+PD3).
In order to improve the optical sensitivity of the photodiode according to the prior art as stated above, it is desirable that at least the entire surface of the cathode region of the photodiode should be covered with a Si.sub.3 N.sub.4 film as an anti-reflection film. In a case where the PN junction boundary between the N-type cathode region and the P.sup.+ -type isolation region is covered with the anti-reflection film of Si.sub.3 N.sub.4, however, surface leakage current crossing the PN junction boundary becomes large compared with the case where it is covered with an insulation film of SiO.sub.2. This is because the interfacial energy levels at the interface between the Si.sub.3 N.sub.4 film and the semiconductor layer are much more than those at the interface between the SiO.sub.2 film and the semiconductor layer.